1. Field of the Invention
The present invention relates to an illumination device and a display device incorporating the same, and more particularly, to an illumination device which provides a uniform light emission distribution at a high efficiency, a back light for a liquid crystal display device, and a liquid crystal display device which incorporates the back light for a liquid crystal display device.
2. Description of the Related Art
In recent years, liquid crystal display devices have been increased in screen size, and have found wider applications in liquid crystal televisions and the like. While the liquid crystal display device consumes lower power than conventional CRT and the like, the power consumed by a back light for the liquid crystal display device (illumination device associated with the liquid crystal display device) has become a growing problem as the screen size has been increased, because the back light consumes a majority of the power consumed by the overall liquid crystal display device. In addition, lower cost is requested for back lights for liquid crystal display devices in order to provide the liquid crystal display devices at lower cost.
As illustrated in FIG. 1, a conventional back light for a liquid crystal display device (liquid crystal BL) comprises a plurality of cold cathode fluorescent lamps (CCFL) 102 arranged side by side; a housing which is covered with a reflective sheet and surrounds the CCFLs; a diffusion plate 101 placed on an emission plane of the housing for uniformizing a light emission distribution; and an optical sheet 103 (lens sheet or the like) laminated on the diffusion plate for converging emitted light. In this structure, light emitted from CCFLs is uniformized while repeating reflections between the diffusion plate and the reflective sheet, and emitted through the diffusion plate. In this event, since the light is absorbed by the diffusion plate, reflective sheet, or CCFLs, only approximately 60% of the emitted light from CCFLs is outgoing through the diffusion plate when the used diffusion plate has a typical transmissivity of approximately 50% and reflectivity of approximately 50%. The resulting low energy efficiency contributes to increased power consumption. Liquid crystal BLs are expensive, because lens shape etc. are formed on a thin sheet and a plurality of optical sheets are used in a back light, which results in an obstacle to reduce the cost of liquid crystal BL.
Specification etc. of Japanese Patent Laid-open Publication No. 2002-352611 discloses a liquid crystal BL which does not employ the diffusion plate as described above (pages 1–3, FIG. 1). The disclosed illumination device has a Fresnel lens arranged on an emission plane to have a high directivity and a relatively uniform intensity distribution. Also, specification etc. of Japanese Patent Laid-open Publication No. 2001-23423 (FIG. 2) discloses a light guide member formed with a blind hole for distributing incident light from a light source from side to side for a uniform distribution of light.
However, the illumination device disclosed in specification etc. of Japanese Patent Laid-open Publication No. 2002-352611 exhibits the uniformity which is merely about 60% of a maximum value across the overall range of a unit Fresnel lens (part of the liquid crystal BL), as illustrated in FIG. 14, and hence suffers from insufficient uniformity.
The illumination device disclosed in specification etc. of Japanese Patent Laid-open Publication No. 2001-23423, in turn, uniformly expands light from a light source through an optical path transformation to make the light nondirectional, and therefore requires light converging means in addition to the light guide member, causing an increase in cost. Further, this illumination device falls under an edge light scheme, and does not take into consideration light incident from out of plane, rather than the side surface of the light guide member. As such, this illumination device cannot be employed as an illumination device generally employed in large liquid crystal display devices, i.e., an illumination device of the type in which light is incident on an uniformizing member (the diffusion plate in FIG. 1) from out-of-plane of the uniformizing member.